In color image processing applications, it is useful to understand how humans perceive colors. By understanding the human visual system and its sensitivity to certain colors, one can more effectively create and manipulate images to create a desired visual effect. This assertion is particularly true in image processing applications that intentionally alter an image to perform a desired function, like hiding information in an image or compressing an image. In digital watermarking, for example, one objective is to encode auxiliary information into a signal, such as an image or video sequence, so that the auxiliary information is substantially imperceptible to humans in an output form of the signal.
Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object, preferably without leaving human-apparent evidence of alteration.
Digital watermarking may be used to modify media content to embed a machine-readable code into the media content. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process.
Most commonly, digital watermarking is applied to media signals such as images, audio, and video signals. However, it may also be applied to other types of data, including documents (e.g., through line, word or character shifting, through texturing, graphics, or backgrounds, etc.), software, multi-dimensional graphics models, and surface textures of objects.
There are many processes by which media can be processed to encode a digital watermark. Some techniques employ very subtle printing, e.g., of fine lines or dots, which has the effect slightly tinting the media (e.g., a white media can be given a lightish-green cast). To the human observer the tinting appears uniform. Computer analyses of scan data from the media, however, reveals slight localized changes, permitting a multi-bit watermark payload to be discerned. Such printing can be by ink jet, dry offset, wet offset, xerography, etc.
The encoding of a document can encompass artwork or printing on the document, the document's background, a laminate layer applied to the document, surface texture, etc. If a photograph or image is present, it too can be encoded.
Printable media—especially for security documents (e.g., banknotes) and identity documents (e.g., passports)—is increasingly fashioned from synthetic materials. Polymeric films, such as are available from UCB Films, PLC of Belgium, are one example. Such films may be clear and require opacification prior to use as substrates for security documents. The opacification can be affected by applying plural layers of ink or other material, e.g., by gravure or offet printing processes. (Suitable inks are available, e.g., from Sicpa Securink Corp. of Springfield, Va.) In addition to obscuring the transparency of the film, the inks applied through the printing process form a layer that is well suited to fine-line printing by traditional intaglio methods. Such an arrangement is more particularly detailed in laid-open PCT publication WO98/33758.
Digital watermarking systems typically have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern by altering data samples of the media content. The reading component analyzes content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reading component extracts this information from the detected watermark. Previously mentioned U.S. patent application Ser. No. 09/503,881, filed Feb. 14, 2000, discloses various encoding and decoding techniques. U.S. Pat. Nos. 5,862,260 and 6,122,403 disclose still others. Of course, artisans know many other watermarking techniques that may be suitably interchanged with the present invention.
One form of digital watermarks is a so-called “fragile” watermark. A fragile watermark is designed to be lost, or to degrade predictably, when the data set into which it is embedded is processed in some manner, such as signal processing, scanning/printing, etc. A watermark may be made fragile in numerous ways. One form of fragility relies on low watermark amplitude. That is, the strength of the watermark is only marginally above the minimum needed for detection. If any significant fraction of the signal is lost, as typically occurs in photocopying operations, the watermark becomes unreadable. Another form of fragility relies on the watermark's frequency spectrum. High frequencies are typically attenuated in the various sampling operations associated with digital scanning and printing. Even a high amplitude watermark signal can be significantly impaired, and rendered unreadable, by such photocopying operations. (Fragile watermark technology and various applications of such are even further disclosed, e.g., in assignee's U.S. patent application Ser. Nos. 09/234,780, 09/433,104, 09/498,223, 60/198,138, 09/562,516, 09/567,405, 09/625,577, 09/645,779, and 60/232,163.).
The present invention discloses a new fragile watermarking technique that is particularly well suited for color imaging applications. A watermark signal in one color plane (or channel) is applied to be out of phase with corresponding watermark signals in other color planes (or channels). An effect of the inventive out-of-phase watermarking technique is to greatly reduce watermark visibility by canceling perceived luminance change in local areas throughout the image. The disclosed watermark is also fragile, since signal-processing operations that combine the out-of-phase color channel with the other channels cancels the watermark signal.
The foregoing and other features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.